1. Field of the Invention
This invention relates to an imagesetter of the "internal drum" type generally used for the production of color separation image masters, and more particularly to a method and apparatus to form and hold the photosensitive material in a cylindrical shape without requiring a rigid cylindrical surface to back up the material.
2. Description of the Prior Art
Conventional laser raster imagesetting devices, as initially produced, created a linear writing line using either spinners or galvos with f-theta field flattening lenses or a holographic spinner with a field lens. This arrangement was combined with a film transport mechanism to move the photosensitive film or paper past the imaging line or station in order to create a two-dimensional image. The film transport mechanism has variously taken the form of a rotated external drum (Omnitech), a flat bed (Hell) where either the bed or the optical system was linearly translated, or more usually a capstan drive (Ultre, ECRM, Linotype and Agfa among others) using a flat platen or a roller platen (part of the capstan system). Devices using this technology were limited in quality by the relatively large writing spot which was obtained due to the optical field angle constraints imposed when forming a relatively long flat writing line. The capstan drive film transport system is subject to slippage, which produces small inaccuracies and non-repeatabilities in image placement. These inaccuracies have restricted the ability of these devices to produce color separation masters, which usually require four registered masters, or to produce printed circuit artwork, which requires high accuracy in image placement.
Other high quality imagesetting devices have utilized an "external drum" technology. In these devices, the photosensitive medium is wrapped around the outside of a drum which can be spun at a fast rate of speed. A writing beam is focused on the surface of the drum, and is transported slowly parallel to the spinning axis (or the writing beam is held stationary and the drum is transported axially) to form the second dimension of scan. These devices achieve an accurate small writing spot, but are relatively high in cost and limited in throughput speed because of the relatively large mass of the drum which must be rotated.
To avoid the problems associated with rotating a heavy drum, devices have been developed that utilize a so-called "internal drum" technology. See, for example, U.S. Pat. Nos. 4,684,228 and 4,712,896. These devices are characterized by having a rigid, heavy partial drum section. Mechanisms are provided to form the photosensitive film or paper to the inside of the drum; these can include vacuum systems and/or computer controlled rollers to precisely push the film against the drum surface. The optical system typically includes a laser light source, either a gas laser or a semiconductor diode laser, a focusing lens, and a monogon spinner: either a 45 degree mirror or a penta or right angle prism. This optical system provides a relatively short focal length (for the writing line length obtained), and therefor a relatively small writing spot size, enabling high resolution images to be produced. The short writing distance also results in less deviation in the uniform placement of the rasters when practical motor/spinner errors are experienced. Further, maintaining the film in a stable position inside the drum during the exposure of the two dimensional image results in highly repeatable image placement, a characteristic that is needed in the production of color separation films for color printing. Greater throughput can be achieved than in the rotating drum systems because higher rotational speeds can be achieved due to the much smaller spinner mass which must be rotated.
These devices are, however, considerably more expensive to produce than the earlier capstan film feed devices because of the massive drum sections which have been employed to create stability at the film plane.
It is desirable to produce an apparatus and method to achieve the benefits of the internal drum technology without incurring the high costs associated with massive drum structures.